Combining Black and White with Color Rendering
My solution was as follows:

Riffing off one of many excellent tutorials by OneMinuteVideoTutorials on YouTube, I used a plane with a transparent material that I linked to a Material ID in node editor.

The first step is assigning the transparent material a Material ID (Material > Settings > Pass Index), and then enabling Material Index passes (Render Layer > Passes > Material Index).

The effect can then be achieved using the following combination of nodes:


Notice that the effect is contingent on the saturation in the Hue Saturation Value node being set to 0.